Is there a relationship to a future temple and what you term the old law?
The temple was destroyed in AD 70.
Ezekiel spoke of a temple. It was not the first or second temple as far as I know. I don't know what it refers to.
Jesus is the temple of God. Jesus puts us in him, so that he can put us in God.
When we are saved, we are given the Spirit of God, we become the temple.
So then, we are seated in heaven with Christ, and Christ lives in our hearts here on earth.
But a discussion of if a person should keep God's Law doesn't need to be tied to if there will be a future temple according to scripture or otherwise, does it?
What do you mean by all saved become Israel?
The first book in the Bible tells us who and what Israel is. See Genesis 32:28.
Genesis 32:28 Then the man said, “Your name will no longer be Jacob, but Israel, because you have struggled with God and with men and have overcome.”
Did you see that? The one who struggles with God and with men and overcomes…
And Jesus in the last book of the Bible confirms this message. See these scriptures in Revelation. Read what Jesus says about overcoming.
Revelation 2:7 He who has an ear, let him hear what the Spirit says to the churches. To him who overcomes, I will give the right to eat from the tree of life, which is in the paradise of God.
And there is only one way a person can overcome this world, and that is through Jesus Christ our Savior.
1 John 5:5 Who is it that overcomes the world? Only he who believes that Jesus is the Son of God.
Therefore, who and what is Israel---Israel is people who overcome by faith in Jesus Christ.